One of the primary goals of genetic engineering has been to control the expression of selected genes in eukaryotic organisms of interest. While it has been relatively straightforward to insert new genes for expression into eukaryotic cells, the targeting of endogenous genes for reduced expression has been more difficult to achieve. Site-directed inactivation of genes in higher organisms has required extremely complex genetic manipulations and is not applicable to a wide range of organisms. One method reducing the expression of specific genes in eukaryotic organisms has been through the use of anti-sense RNA and through co-suppression.
Anti-sense RNA has been used to reduce the expression of pre-selected genes in both plants and animals. Descriptions of the use of anti-sense RNA to reduce the expression of selected genes in plants can be found, among other places in U.S. Pat. No. 5,107,065, Smith et al. Nature 334: 724-726 (1988), Van der Krol et al., Nature 333: 866-869 (1988), Rothstein et al., Proc. Natl. Aca. Sci. USA 84:8439-8443 (1987), Bird et al., Bio/Technology 9:635-639 (1991), Bartley et al. Biol. Chem. 267:5036-5039 (1992), and Gray et al., Plant Mol. Bio. 19:69-87 (1992).
Another method of reducing the expression of specific genes in eukaryotic organisms is through the use of co-suppressor RNA. Co-suppressor RNA, in contrast to anti-sense RNA, is in the same orientation as the RNA transcribed from the target gene, i.e., the "sense" orientation.
It is possible that biochemical pathways in plants transfected with hybrid viruses could be altered by overproducing an enzyme involved in a rate-limiting step, or by inhibiting the synthesis of an enzyme via antisense RNA. Although the expression of numerous genes in transgenic plants have been repressed by antisense RNA, the actual mechanism and location of inhibition is not known. In the nucleus, antisense RNA may directly interfere with transcription or form duplexes with the heterogeneneous nuclear (hnRNA). There is evidence that inhibition of endogenous genes can occur in transgenic plants containing sense RNA A. R. van der Krol et al., Nature 333:866-869 (1988) and C. Napoli et al., Plant Cell 2:279-289 (1990) mechanism of this down regulation or "co-suppression" is thought to be caused by the production of antisense RNA by read through transcription from distal promoters located on the opposite strand of the chromosomal DNA (Greison, et al. Trends in Biotech. 9:122-123 (1991)). Alternatively, in the cytoplasm, antisense RNA may form a double-stranded molecule with the complimentary mRNA and prevent the translation of mRNA into protein.
Tobamoviruses, whose genomes consist of one plus-sense RNA strand of approximately 6.4 kb, replicate solely in the cytoplasm, and can be used as episomal RNA vectors to alter plant biochemical pathways. Hybrid tobacco mosaic (TMV)/odontoglosum ringspot viruses (ORSV) have been used previously to express heterologous enzymes in transfected plants (Donson, et al. Proc. Natl. Aca. Sci. USA 88:7204 (1991) and Kumagai, et al. Proc. Natl. Aca. Sci USA 90:427-430 (1993), minus-Sense RNA Strand, (Miller, et al.). Infectious RNA transcripts from viral cDNA clones encode proteins involved in RNA replication, movement, and encapsidation (10). Subgenomic RNA for messenger RNA synthesis is controlled by internal promoters located on the minus-sense RNA strand (N.benthamiana plants were inoculated with in vitro transcripts as described previously W. O. Dawson, et al., Proc. Natl. Acad. Sci. U.S.A. 83, 1832 (1986)!). Insertion of foreign genes into a specific location under the control of an additional subgenomic RNA promoter have resulted in systemic and stable expression of neomycin phosphotransferase and .alpha.-trichosanthin (Donson, et al. Proc. Natl. Aca. Sci. USA 88:7204 (1991) and Kumagai, et al. Proc. Natl. Aca. Sci USA 90:427-430 (1993)).
One of the many biochemical pathways that could serve as a target for genetic manipulation is the biosynthesis of carotenoids. On the first committed step in carotenoid biosynthesis in higher plants is the condensation of two geranylgeranyl pyrophosphate molecules to phytoene, a colorless C.sub.40 hydrocarbon, by the enzyme phytoene synthase. In the ripening fruit of Lycopersicon esculentum, phytoene synthase is a monomeric, chloroplast localized protein with an approximate relative molecular mass of 42 kDa. This enzyme is initially synthesized as a 47-kDa preprotein and is processed by the removal of a transit peptide during import to the chloroplast (Bartley, et al. J. Biol. Chem. 267:5036-5039 (1992)). Transgenic tomato plants containing anti-sense to phytoene synthase mRNA produce yellow fruit and pale flowers. Although the fruit specific carotenes are reduced by 97%, the levels of carotenoids in the leaves of the transgenic plants are unaffected, (Bird, et al., Bio/Technology 9:635-639 (1991)). It has been proposed that an additional set of biosynthetic genes occurs in plants which regulate the expression of leaf specific carotenoids.
The subsequent step in the biosynthetic pathway is the modification of the colorless phytoene to phytofluene and .zeta.-carotene by phytoene desaturase. Among higher plants, the isolation of gene encoding this enzyme has been described for tomato, Pecker, et al., Proc. Natl. Acad. Sci. U.S.A., 89, 4962 (1992), and Arabidopsis thaliana (Scolnick and Bartley, Plant Physiol 103:147 (1993)). Phytoene desaturase is inhibited by norflurazon, a bleaching herbicide, in a reversible, non-competitive manner (Sandman, et al., Target Sites of Herbicide Actions, G. Sandman, P. Boger Es. (RC press, Boca Rotan (1989)). Application of this compound causes a dramatic decrease in leaf carotenoids and chlorophylls and a subsequent accumulation of phytoene. The reduction of the photoprotective carotenoids derived from phytoene may cause a rapid destruction of chlorophyll by photooxidation.
The need for new methods of reducing the expression of specific genes in eukaryotes is clearly established. The invention described herein provides new methods for reducing the expression of selected genes, genetic constructions for practicing the methods, and cells transformed by these genetic constructions, and higher organisms comprising the transformed cells.